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Bluegill
01-14-2013, 08:26 AM
How long (in time, hours) could I run a 9.5 HP, year 1968, Johnson outboard at full throttle on 3 gallon of gasoline ???
I have a 6 gallon gas tank and I am concerned about the long gas remains in the tank.
I think a 3 gallon tank would be much more than adequate.

Ian McColgin
01-14-2013, 08:37 AM
Those older engines burn, best I recall, more than a gallon an hour. Stick with the 6 gallon and log of you use in three or four hour runs to be sure.

Rich Jones
01-14-2013, 09:11 AM
I had a 1959 35hp Gale engine that went thru gas at an unbelieveable rate. If you use the boat alot, stick with the 6 gallon and add fuel stabilizer. I use a 3 gallon tank for my 15hp 4-stroke and it lasts from May-September, but the 4-stroke uses very little gas as compared to an old 2-stroke.

Ian McColgin
01-14-2013, 09:51 AM
And a huge difference between staying throttled back for easy displacement speed or throttled up to travel. The problem with some of those older engines is that they really fould their plugs if not run flat out a bit.

Eddiebou
01-14-2013, 12:18 PM
I've got the same motor on a 13 1/2' plywood skiff and how long can you run it?- Longer than you want to hold the tiller for sure. A 3 gal tank is more than adequate for me, just taking short little fishin' trips. But if I already had a 6 gal tank, that's what I'd use. I mix fuel in relation to what I'm gonna use about 1 to 2 gal at a time so it's not sittin around.

Kudzu
01-14-2013, 02:25 PM
...... I mix fuel in relation to what I'm gonna use about 1 to 2 gal at a time so it's not sittin around.

I was going to say just mix 3 gallons in a 6 gallon can. I would rather have the extra capacity just in case.

BC Bill
01-14-2013, 07:06 PM
I had a 9.8 HP Merc on a 12-FT MFG fiberglas skiff. 3 gal tank would get me down to Barnegat Inlet and back which IIRC was around 25 miles round trip.

Being a kid, I generally ran wide open. But I always carried a 5-gallon emergency can of gas for a back-up!

---Bill

Ian McColgin
01-14-2013, 07:08 PM
About right since that's under two hours running time.

Neddie Seagoon
01-15-2013, 12:17 AM
Oh dear, you've stumbled upon a thermodynamicist here, so I'm gonna hafta chime in. This is a pretty simple calculation actually.

There are 120,000 BTU in a gallon of gasoline. 1 Horsepower is 2544 BTU/Hr so that's 47.17 HP*Hr / gal. Divide by 9.5 HP to get 4.96 Hr/gal. This assumes all the heat gets converted to Horsepower. It doesn't. In a really good gasoline engine you can get efficiencies around 32% You say this is an old motor so lets assume 25% 5Hr/gal times 25% gives about an hour and a quarter per gallon. Your milage may vary. :ycool:

willmarsh3
01-17-2013, 08:54 PM
That seems about right. I could go about 25 miles or so in a Catalina 25 (4000 lbs displacement) on 6 gallons.

Bluegill
01-19-2013, 07:30 AM
That seems about right. I could go about 25 miles or so in a Catalina 25 (4000 lbs displacement) on 6 gallons.

How many hours ???